Engine exhaust systems utilize emissions control devices to treat exhaust gas of internal combustion engines in order to reduce the amount of particulate emissions released to atmosphere. Emission control devices include catalytic converters, such as three way catalytic converters (TWC) capable of reducing NOx and other pollutants. Emission control devices may further include particulate filters (PFs) positioned downstream of the TWC to collect particulate matter, such as carbon particles from incomplete combustion. Exhaust gas sensors may be coupled to the emission control device to monitor the flow of exhaust and to control the air-fuel ratio (AFR) of exhaust near stoichiometry. The exhaust gas sensors may include various oxygen sensors, such as oxygen sensors coupled upstream and downstream of the catalytic converter. Other sensors may include temperature and pressure sensors. Emissions regulations require on-board diagnostic routines to be regularly performed to ensure that the various emissions control devices and the associated sensors are functioning.
One example approach for diagnosing the emission control device catalyst is shown by Santillo et al. in U.S. Pat. No. 9,359,967. Therein, during feedback engine air-fuel ratio control, responsive to a downstream catalyst exhaust gas sensor, degradation of the catalyst may be indicated in response to a catalyst transfer function determined within a specified frequency range, the range based on the exhaust gas sensor output. In other words, catalyst degradation may be determined following small air-fuel ratio variations that are otherwise routinely used to increase catalyst efficiency. In other approaches, catalyst degradation is determined based on the exhaust sensor output following a targeted significant perturbation.
However, the inventors herein have recognized potential issues with such systems. As one example, the approach of U.S. Pat. No. 9,359,967 relies on a fully functional exhaust gas sensor measurement to assess potential degradation of the catalyst. While other on-board diagnostic routines may indicate if the exhaust gas sensor is functional or degraded, such routines typically assess the output of the same sensor during selected conditions. However, since the output of the catalyst exhaust gas sensor are themselves significantly impacted by air-fuel ratio perturbations, there may be conditions when the sensor is determined to be degraded when it is functional. For example, if there is an exhaust air leak upstream of the exhaust gas sensor, the output of the sensor may inadvertently reflect a slow sensor response, even though the sensor is not degraded. Since the output of the sensor is relied on to diagnose the upstream catalyst, errors in sensor status can result in significant errors in catalyst status. For example, an inaccurate assessment of a slow sensor response can result in a false-pass of a catalyst monitor, resulting in emissions issues.
In one example, the issues described above may be addressed by a method for an engine, including: adjusting a diagnostic threshold of an oxygen sensor coupled downstream of an exhaust catalyst responsive to a measured pressure at a particulate filter coupled downstream of the oxygen sensor. In this way, the effect of an exhaust air leak at a downstream particulate matter filter may be used to assess the functionality of an upstream oxygen sensor, and adjust the execution of an on-board diagnostic routine.
As an example, responsive to entry conditions for a monitor for an exhaust oxygen sensor coupled downstream of an exhaust catalyst being met, a pressure associated with a particulate matter filter, coupled downstream of the oxygen sensor, may be assessed. If the pressure is indicative of an exhaust air leak, such as may occur when the pressure measured upstream or across the filter is lower than a threshold pressure, a threshold for the oxygen sensor monitor (herein also known as a CMS monitor) may be adjusted (e.g., lowered). The monitor is then executed with an output of the oxygen sensor compared to the adjusted threshold. Once the sensor monitor is executed reliably, a catalyst monitor which depends on the output of the sensor can be assessed.
In this way, an exhaust air leak that may corrupt the output of an oxygen sensor may be timely and reliably identified based on the output of a pressure sensor coupled to a downstream particulate matter filter. By reliably identifying an exhaust air leak, a degraded sensor response that is due to the effect of the exhaust air leak may be better differentiated from an actually degraded sensor response. By adjusting a diagnostic threshold for an oxygen sensor monitor based on a pressure response at a downstream particulate matter filter, the reliability of an oxygen sensor monitor may be improved. In turn, the reliability of a catalyst monitor, that uses the output of the oxygen sensor, can be improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.